1. Field of the Invention
The present invention generally relates to a digital rolloff filter used in a digital radio communication system. More particularly, the present invention pertains to a digital rolloff filter employed in a QPSK (Quadrature Phase Shift Keying) modulator.
2. Description of the Related Art
In a digital microwave communication system, a waveform-shaping filter (rolloff filter) is required in a modulator and a demodulator, respectively. Very recently, digital filters for filtering digital signals on a time base of a base band are practically utilized, since there are great progressive steps in the digital signal processing techniques, the operation speeds of devices, and the higher integration of devices. Thus, digital filters without characteristic fluctuations, aging effects, and also temperature changing problems can be realized.
There are two typical digital filters, i.e., an IIR (Infinite Impulse Response) type digital filter and an FIR (Finite Impulse Response) type digital filter. In the digital microwave communication system, an FIR type digital filter capable of realizing a linear phase is employed.
This conventional FIR type digital filtering system will now be explained with reference to a drawing.
FIG. 1 is a schematic block diagram for representing 1 channel of a QSPK-transmitter end rolloff filter constructed of the conventional FIR type digital filter.
In this rolloff filter, a series of transmit data is inputted into a terminal 81, and passes through a shift register 151 constructed of a plurality of D flip-flops designated by D. The outputs from the respective registers are entered into taps (multipliers) 411 to 416 to be multiplied by tap coefficients. The outputs from the respective taps (multipliers) 411 to 416 are entered into an adder 311 so as to be added with each other. That is, all of the tap outputs are fully added in this adder 311, which is connected to terminal 82. At this time, the sampling values of the impulse responses corresponding to the frequency characteristic of the digital filter become tap coefficients "Cj" of the respective taps (multipliers). Note that symbol "j" indicates integers from -N to N in the case of (2N+1) taps. Assuming now that the data present in the shift register is "ak-j", an output "bk" of the digital filter is given by the following equation (1): ##EQU1## Thus, such a frequency characteristic corresponding to the discrete Fourier transform of the tap coefficient Cj. When the number of taps is increased up to infinite, arbitrary frequency characteristics may be realized.
The above-explained conventional FIR type digital filter is described in various publications, for example, "Digital Filters theory and applications" written by N. K. Bose published by North-Holland.
However, in the case that a sharp or steep frequency characteristic is realized in the FIR type digital filter, the convergence characteristic of the impulse response thereof would be deteriorated. In other words, since the integer N becomes very large in such a manner that the tap coefficient "Cj" becomes negligibly small in fact, a large number of taps would be required. For instance, both the taps (multipliers) Cj and the adder 311 may be arranged by such circuits as shown in FIG. 2 and FIG. 3. That is, since the multipliers of FIG. 2 are 1.times.n bits multiplication, these multipliers may be realized by employing simple logic gates 571 and 561 to 563. However, since the adder shown in FIG. 3 is arranged by combining (a total tap number--1) pieces of two-input full adders, 361, 362, 363, this circuit arrangement owns a drawback such that when the total tap number is large, the resultant circuit scale would be increased, i.e., bulky circuit arrangement.
To solve this drawback, another conventional method has bee proposed in which the FIR type digital filter is constituted by the ROM. That is, all data "ak-j" present in the shift registers are defined in correspondence with the addresses of the ROM, and an output "bk" of the digital filter with respect to an input signal to this digital filter is previously calculated.
Then this precalculated value is inputted to the data of the ROM, so that the FIR type digital filter may be constituted by employing a single ROM. Moreover, this conventional method has other drawbacks. That is, due to limitations in the operation speed of this ROM and also in the address bit number, this conventional method may be applied only to such a modulator having the rolloff ratio of approximately 0,4, the modulation speed of which is relatively low, and further the band limitations of which is not so severe.